1. Technical Field
The present invention relates to a small size, light weight battery pack which has a flat battery contained in a container.
2. Background Art
With an ever increasing reduction in size and weight of a variety of electronic devices such as portable telephones and video cameras, a reduction in size, thickness and weight as well as improved battery characteristics including a higher energy density have been required to power supplies for driving electronic devices, such as Li ion secondary batteries.
The Li ion secondary battery is structured such that electrodes and electrolyte are encapsulated in a battery can in order to prevent the electrolyte from leaking. For meeting the requirement of a reduction in thickness, the battery can must be reduced in thickness dimension. However, it is difficult to fabricate such a thinner can by drawing process. On the other hand, for fabricating a thinner can by welding, a battery can must be seal-welded, thereby causing a lower production efficiency and a higher cost.
To solve the problem as mentioned above, film type secondary batteries (flat battery cells) have been developed by using a film of polymer solid electrolyte as electrolyte. One example of such a battery cell is illustrated in FIG. 21.
The flat battery cell illustrated in FIG. 21 includes an electric power generating element A comprised of a positive electrode; a negative electrode; and an electrolyte 3 disposed between the two electrodes. The positive electrode has an Al foil 1 of approximately 20 to 40 .mu.m in thickness, one side of which is coated with a positive electrode mixture 1b having LiCoO.sub.2 as an active material, except for a positive tab 1a. The negative electrode has a Cu foil 2 of 20 to 40 .mu.m in thickness, one side of which is coated with a mixture 2b including a carbon material except for a negative tab 2a. Also, the electrolyte 3 is formed of a gel electrolyte including a film type polymer matrix which is swelled by a Li-ion containing electrolyte.
The electric power generating element A is wrapped by two sheets of armor materials 4, for example, Al laminate films, where peripheral portions of the armor materials 4 are thermally fused. In FIG. 22, reference numerals 4A.sub.1, 4A.sub.2, 4A.sub.3, 4A.sub.4 designate four sealings of the armor materials 4.
The illustrated flat battery cell has a thickness of approximately 2.5 to 5 mm, and the sealings 4A.sub.1 to 4A.sub.4 has a width of approximately 3 to 8 mm.
Upon sealing the armor materials 4, as both the armor materials are thermally fused and simultaneously applied with equal upward and downward pressures, a flat battery cell B.sub.1 as illustrated in FIG. 23 is fabricated, with the positive and negative tabs 1a, 2a drawn out from a central position in the thickness direction of the battery cell. Alternatively, when both the armor materials are thermally fused while one of the armor materials is only applied with a pressure, a flat battery cell B.sub.2 as illustrated in FIG. 24 is fabricated, with the positive and negative tabs 1a, 2a drawn from a position which is offset to the other armor material.
Since these armor materials are readily damaged, the flat battery cell is generally served in the form of a battery pack which contains the flat battery cell in a container made of a resin.
A battery pack 200 illustrated in FIG. 25 contains the flat battery cell B.sub.1 or B.sub.2 within a resin-made container 100 which has a divided structure consisting of an upper case 100a and a lower case 100b. For fabricating the battery pack 200, the flat battery cell is disposed in the lower case 100b with the sealings 4A.sub.1, 4A.sub.2, 4A.sub.3 bent upward or downward, and a circuit board 6 mounted with parts of a protection circuit for monitoring a battery voltage and a battery temperature and for controlling charging and discharging currents is accommodated in the lower case 100b. Then, terminals 6a of the circuit board 6 are electrically connected to the positive and negative tabs 1a, 2a of the flat battery cell drawn from the sealing 4A.sub.4. Next, the upper case 100a is mated with the lower case 100. In FIG. 25, reference numeral 6b designates a molding which molds circuit parts with a resin, and 6c designates a lead set which includes a positive electrode lead, a negative electrode lead and a temperature sensor lead.
As previously described, battery packs are required to have a higher capacity, a smaller size and a lighter weight. Thus, the battery packs are simultaneously required to have flat battery cells of higher capacity and to be reduced in thickness and two-dimensional size of the battery pack.
The thickness of a battery pack is defined by the overall thicknesses of a flat battery cell, a circuit board and a container. Since the thickness of the flat battery cell is increased as it has a larger capacity, the container must be reduced in wall thickness for simultaneously achieving a higher capacity and a smaller thickness of the battery pack. However, a conventional resin-made container has a limitation to a reduction in wall thickness of the container from a viewpoint of ensuring a sufficient strength of the container. For example, it is difficult to reduce the wall thickness of the upper and lower cases, which form the container, to approximately 0.4 mm or less.
The two-dimensional size of the battery pack is mainly defined by the two-dimensional shape of an electric power generating element of the flat battery cell, the two-dimensional shape of the circuit board, and the two-dimensional area of a connection region 7' between the positive and negative tabs and the circuit board. Here, the two-dimensional sizes of the electric power generating element and the circuit board are defined basically by requirements to the characteristics of the battery cell, so that it is difficult to reduce the two-dimensional sizes of the electric power generating element and the circuit board from a viewpoint of simultaneously achieving a higher capacity and a smaller size of the battery pack.
Generally, parts of a protection circuit mounted on a circuit board of a battery pack provide an excessive charging protection function which controls a charging current to prevent the battery from being excessively charged, and an excessive discharging protection function for preventing a polarity change due to excessive discharging, an increased internal pressure, deteriorated performance, and so on. In addition, some protection circuits also provide an excessive current protection function for protecting the battery from an excessive current which may be generated when the battery is externally short-circuited.
Recent improvements in the performance of flat battery cells result in less requirements to the excessive charging protection function, and the excessive discharging protection function can be eliminated by reducing a leak current of a battery. On the other hand, however, the excessive current protection function is still needed since external short-circuiting can occur irrespective of the performance of a particular battery, and therefore a current fuse has been conventionally used for protecting the battery from an excessive current.
The current fuse, however, is designed to protect a battery from an excessive current as it is fused by resistive heat which is generated when the excessive current is generated, so that the current fuse involves a problem that it loses the excessive current protection function after it is fused.
For connecting a flat battery cell and a circuit board, positive and negative tabs of the flat battery cell are generally laid on the top of a positive electrode land and a negative electrode land on the protection circuit board, and are connected to each other by spot welding or ultrasonic welding. Generally, the lands on the circuit board are formed of Cu, whereas the positive tab of the flat battery cell is made of Al and the negative tab is made of Ni.
In this way, the positive electrode land and the positive tab are made of different types of materials from each other, and likewise the negative electrode land and the negative tab are also made of different materials from each other. For this reason, the strength of the connection between the lands and tabs are not so high, causing the connection structure to be susceptible to failure when the battery pack is applied with an impact, and possibly the loss of the battery function.
As previously described, a flat pack may be equipped in a portable telephone, and may be removably mounted, for example, into the back of a telephone body. Generally, a flat battery cell has a plurality of, for example, three flat terminals which are exposed on the surface of a container, in which the flat battery cell is contained, through holes formed therethrough. As the battery pack is mounted into the telephone body, these flat terminals are brought into face contact with three corresponding flat terminals which are exposed on a surface of the telephone body on which the battery pack is mounted, thereby resulting in electrical connection of the flat battery cell to a power supply circuit of the portable telephone. For a personal digital assistant (PDA), likewise, a plurality of, for example, six flat terminals of a battery pack are brought into face contact with six corresponding flat terminals of the PDA.
As described above, while the wall thickness of the container is reduced to meet the requirements of a higher capacity and a reduced thickness to the flat battery cell, the flat battery cell is more susceptible to flexure and/or deformation as the wall thickness of the container is smaller, when an electronic device is applied with an external force by some reason. This is particularly true in the structure in which electric connection is established between a battery and an electronic device such as a portable telephone, PDA or the like through flat terminals of both sides which are brought into face contact with each other. The flexure and/or deformation, if any, causes the breaking of a normal face contact state between the flat terminals on the battery side and the flat terminals on the telephone body side, thus resulting in a failed contact between both sides.
In addition, since the flat terminals are exposed, inadvertent storage of the battery pack into a bag or the like, by way of example, may cause the flat terminals to contact any metal article and consequently result in external short-circuiting. Furthermore, the surfaces of the flat terminals may be contaminated by sweat or the like to deteriorate a conduction with the corresponding terminals when the battery pack is mounted into the telephone body.